Inverse feedback amplifier



June 17, 1941. P, F G, HOLST UAL' 2,245,671

INVERSE FEEDBACK AMPLIFIER Filed Aug. 31, 1939 2 Sheets-Sheet 1 June 17,1941.

P. F. G. HoLsT E-rAl.

INVERSE FEEDBACK AMPLIFIER Filed Aug. 51, 1939 2 Sheets-Sheet 2 GttornegPatenied June 17, 1941 INVERSE FEEDBACK AMPLIFIER Paul F. G. Holst andLoren R. Kirkwood, Oaklyn,

N. J., assignors to Radio Corporation of America, a corporation ofDelaware Application August 3l, 1939, Serial No. 292,930

11 Claims.

The present invention relates to inverse feedback amplifiers, and hasfor its primary object to provide an improved inverse feedback amplifierfor audio frequency signal amplification in radio signal receiving andphonograph record reproducing apparatus and the like, which may includea phase inverter stage and impedance coupling with a balanced outputamplifier stage in the feedback loop without introducing objectionablephase shift in the signal transmitted through the amplifier Within thepass band of the amplifier.

It is also a further object of the invention to provide an amplifier ofthe type referred to, wherein a tendency to oscillate and overload atfrequencies above and below the useful audio frequency range isprevented.

As is well known, inverse feedback to produce degeneration inamplifiers, particularly of the audio frequency type, is accomplished bycoupling back from one stage to a preceding or input stage, a portion ofthe output energy. The phase relationship and relative magnitudes of thefeedback and input voltage are the most important considerations. Thephase relation of the energy fed back must be such that degeneration voccurs, and, for this condition, the phase relation between the feedbackand input voltages must be such that said voltages are approximately 180degrees out of phase.

if, for any reason, the phase relationship above referred to is causedto shift to such an extent that it approaches 180 degrees therefrom ineither direction, a condition of regeneration occurs and may result inoscillation, in which case the amplifier becomes inoperative fortransmitting normal signal energy.

Accordingly, it is also an object of the present invention, to providean improved inverse feedback amplifier including a phase inverter stagehaving a coupling network including means for preventing high and lowfrequency oscillation outside of the useful range of signal frequencies,thereby preventing an oscillating condition from being initiated wherebythe amplifier may be rendered inoperative.

It is a still further object ofthe present invention to provide an audiofrequency amplifier having a driver stage and a phase inverter stage forconverting a single-ended input circuit into a push-pull orbalancedoutput circuit for driving an output amplifier of the balancedor push-pull type, without the use of a coupling transformer in thecoupling network, and without introducing appreciable phase distortion.In a push-pull or balanced -amplifier stage, the signal voltage appliedto the grids of the push-pull or balanced tubes must be substantiallydegrees out of phase, and substantially equal in amplitude. Theelimination of a coupling transformer greatly simplifies the applicationof inverse feedback because `of the elimination of a source of phaseshift which may cause a possible 180-degree phase shift over the audiofrequency range.

It is well known that audio frequency amplifiers employing pentodeoutput tubes, while highly efcient, are subject to distortion resultingfrom the pentode tubes operating into a load which varies in impedancewith frequency. It is, therefore, a further object of the presentinvention to provide an inverse feedback amplifier circuit in connectionwith a balanced pentode amplifier stage for driving a loudspeaker,wherein the effect above referred to has been minimized or eliminatedwithout the use of the usual anode circuit or primary circuit loadings.

It is also an object of the present invention to provide an amplifierwhich tends to damp out motional impedances of the loudspeaker voicecoil refiected over into the output circuit of the amplifier stage.

It is a still further object of the present invention to provide aninverse feedback amplifier having a feedback loop comprising two stagesin one branch of the loop and three stages in the other branch thereofin which overall phase distortion is less than the phase distortionthrough the three-stage circuit.

As is well known, phase distortion results when the phase relationbetween the input and output signal voltages are changed in passingthrough the amplifier, this phase shift ordinarily being variable withfrequency.

Phase distortion ordinarily is relatively unirnportant in 'audiofrequency amplifiers as the phase relation of the various voltages in acomplex wave may be shifted over a considerable r-ange withoutnoticeable effect upon the ear. However, in inverse feedback amplifiers,the overall phase distortion is highly important in that a shift in thephase relation established in the circuit by an amount approaching 180degrees may result in serious regenerative effects and oscillationtending to render such amplifiers inoperative, as hereinbeforementioned. In fact, the damage may in some cases involve burn-out of thecircuit elementsl and of the tubes constituting the output stage.

Therefore, it is an object of the present invention to provide means forstabilizing an inverse feedback circuit as part of a coupling networkbetween a single-ended input circuit and a push-pull or balanced outputcircuit, which prevents the transmission of signal frequencies throughthe inverse feedback loop which tend to cause regeneration andoscillation.

It is also an object of the present invention to provide, in conjunctionwith an inverse feedback amplifier of the character referred to, a tonecontrol system which is outside of and not included in theY feedbackloop or either branch thereof, whereby effective tone control is madepossible with an inverse feedback amplifier.

In providing a tone control circuit of this type in a preferredembodiment of the invention, the selectivity of the intermediatefrequency amplifier portion of the receiving system is made adjustableor variable independent of the control of the frequency characteristicprovided by the inverse feedback amplifier, and with or withoutadditional tone control means in conjunction with volume Controlelements in the receiving system, also outside of the inverse feedbackloop and either branch thereof.

The invention will be better understood from the following description,when considered in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

In the drawings,

Figure 1 is a schematic circuit diagram of a radio receiving systemprovided with an amplifier and tone control means therefor embodying theinvention;

Figure 2 is a schematic representation of a portion of the circuit ofFigure 1 rearranged to illustrate an operating characteristic thereof;

Figure 3 is avector diagram of certain signal voltages indicated inFigs. 1 and 2;

Figure 4 is a further schematic representation of a portion of thecircuit of Figure 1 illustrating a further operating characteristic ofthe circuit thereof; and

Figures 5 and 6 are graphs showing curves 1ndicating the responsecharacteristic of the tone control portions of the system.

Referring to Fig. 1, the radio receiving system shown by way of exampleis of the superheterodyne type comprising a radio frequency amplifier,detector and oscillator portion together with suitable tuning meansrepresented by the rectangle 5 which is coupled through a suitableintermediate frequency amplifier 6 and control system 1 with a seconddetector and audio frequency amplifier 8,

The two-stage intermediate frequency amplifier ii-Iii illustrated isprovided with two tuned intermediate frequency input circuits II and I2in which are connected additional coupling windings i3 and I4 undercontrol cf selector switches l5 and I5 which operate in steps to connectinto the tuned circuits i I and I2 a portion of or the total additionalcoupling windings I3 and I4 as selectivity adjusting or varying means toincrease or decrease the fidelity, as will hereinafter be referred to.The selectivity adjusting means represents any suitable means for thispurpose.

The second detector 3 includes a diode rectifier anode I8 and groundedcathode I9, the latter being common with an amplifier portion in thesame envelope, comprising a control grid and an output anode 2l. Thedetector diode is connected with the grounded output resistor 22 forrectified signals which are transferred through a resistor 23 to avolume control network including a volume control potentiometer 24-25 towhich the control grid 25J of the amplifier portion is coupled through acoupling capacitor 26 and a grid resistor 21. The volume control networkalso includes tone control or compensating shunt circuits 28 and 29connected with the volurne control element 24 for adjusting the tone orfidelity with variations in volume, as is Well known. The signal outputfrom the diode detector IS-IB is amplified in the amplifier portionI9-2il-2I and the amplified signals are applied through an outputcircuit 35 to the remainder of the audio amplifier system through aYsuitable audio frequency filter 3l preferably adjusted to cut off 10 kc.beat notes, the single-ended output from the detector and amplifierbeing derived across the output resistor 32 in the anode circuit 30 inthe amplifier portion of the tube 8.

The audio frequency signals appearing across the output resistor 32 areapplied to a driver stage comprising a tube 35 through a suitablecoupling means such as a capacitor 36 and appear as the voltage e acrossthe grid resistor 31 and a bypass capacitor 38 to ground in theinputcircuit of the amplifier 35.

The input circuit, including the resistor 31, is connected to the inputgrid 39 of the amplifier and to a source of fixed biasing potentialindicated at 4i) through a supply lead 4I and a filter resistor 42provided with a filter capacitor 43, one side of the potential source 40being grounded. The bypass capacitor 38 is of relatively low impedanceto audio frequency signals so that the lower potential end of the inputcircuit or resistor 31 is substantially at ground audio frepuencypotential. The input signal voltage e is applied to the grid of the tube35 and the amplified output voltage of this tube is applied through acoupling capacitor 45 to the signal input grid 4B of one tube 41 of apair of balanced oiitut tubes, the other of which is indicated a Theamplified output voltage from the tube 35 is also applied to a signalpotentiometer or bleeder circuit comprising' a lead 43, a blockingcapacitor 50 and two bleeder or potential divider resistors 5I and 52connected serially, as indicated, between .the output circuit 53 and thefilter resistor 42 for the bias supply source at a terminal indicated at54.

The capacity of the blocking capacitor 5E) is such that it offersrelatively low impedance to audio frequency .signals so that the portionof the signal voltage that exists across the resistor section 52 of thepotentiometer is applied to a signal input or control grid 55 of a tube56 through a tap connection 51 between the resistor sections 5I and 52.The tube 5S provides the phase inverter stage of the amplifier forderiving a balanced signal output for the tubes 41 and 48 from asingle-ended input circuit 31, the arrangement being such that theconnection 51 for thecontrol grid 55 of the inverter stage on the signalpotentiometer or bleeder is at a point which provi-des a signal voltageon the grid 55 of the same value as the signal voltage applied to thecontrol grid 39 of the driver stage 35. For this purpose, the resistancevalues of the resistor sections 5I and 52 are adjusted to providesubstantially unity gain between the plate circuit 53 and the grid 55 ofthe output tube 48 through the impedance coupling network comprisingplate resistors 60 and 6I, -grid resistors 62 and 63 and the couplingcapacitor 64 between the inverter anode 56 and the grid 65.

The grid circuits for the output tubes 41 and 43 are provided with asuitable bypass for audio frequencies to ground by a capacitor indicatedat B, and bias potential is applied to the control grids of the outputtubes through a suitable connection including a filter resistor 61.

The output voltage from the inverter tube 55 is inverted or 180 degreesout of phase with respect to the output voltage from the tube 35.Consequently, the voltages e1 and e2 applied to the grids of thepush-pull tubes 41 and 48 are substantially 180 degrees out vof phaseand equal as required for push-pull or balanced operation.

The M30-degree phase inversion is present in any normal tube withresistive load and results from the fact that, when the grid of the tubeswings positive, the plate current increases and the A. C. component ofthe plate voltage swings negative. Thus a positive half cycle applied tothe grid produces a negative half cycle of plate voltage and vice versa.Advantage is taken of the fact in providing resistance coupling betweenthe driver and 4phase inverter tubes and the power output tubes 41 and48. Thus the signal voltage applied to the input circuit of the outputtube 41 is in phase with the signal voltage applied to the grid of theinverter tube 55 and, as the phase is through the inverter tube, thevoltage e2 which is applied to the output tube 48 is 180 degrees out ofphase with respect to the voltage 6i.

The output circuit 68-59 of the balanced amplifier' 41-43 is connectedin balanced relation between the output anodes 10 and 1| and cornprisesa primary 12 of an output transformer 13 having a secondary winding 14connected with the operating or voice coil winding 15 ofa loudspeaker16. The winding indicated at 11 is the hum-bucking winding for theloudspeaker. y Anode potential is supplied to the output circuit througha center tap 19 on the primary of the output transformer as normallyprovided in such circuits. However, the primary circuit is devoid ofloading resistors and capacitors as normally provided to stabilize theamplifier and to prevent distortion occasioned by the reflectedloudspeaker load in the output circuit providing a load characteristicwhich varies' with frequency. This is for the reason that the effectiveoutput impedance of the tubes 41 and 48 is lowered by the inversefeedback connection which includes a portion di) of the output secondary14 between one end thereof which is grounded as indicated at Si and atap 82 included in circuit with the cathode E3 of the input or driverstage 35 through the connection therewith provided by the externalinverse feedback circuit 84 and the ground connection 8l.

In this manner. a portion of the output voltage existing across thesecondary 14 between the terminal 82 and ground is applied in seriesbetween tlie cathode 83 and ground to the driver stage 35 and inopposition to the input signal voltage e between ground and the controlgrid ,39 across the resistor 31. These voltages are substantially 180degrees out of phase and the result is a degree of degenerative feedbackdependent upon the voltage derived from the winding 14, and may beincreased to over 15 db. without danger of oscillation.

With this connection, pentodes for high efciency amplication may beutilized at 41 and 48 in the output stage. As the load impedance shiftedsubstantially 180 degrees increases with frequency, the feedback voltagein-V creases because of the lighter load, tending to cut down theamplification with increased frequency and thereby providing asubstantiallyilat overall response. This has the same effect as loweringthe output impedance of the output stage. This in turn tends tostabilize the operationof the amplifier; The motional impedance of theloudspeaker-operating coil reflected back into the output circuit isdamped out by the feedback action without requiring any loadingresistors or capacitors across the primary winding for this purpose, andwithout the attendant losses in signal strength which otherwise must beaccepted.

It will be noted that the inverse feedback loop through the amplierexten-ds from the output vwinding 30 from which the feedback potentialisderived to the input circuit of the driver stage 35 in series with theinput potential provided across the resistor 31 through ground and thebypass capacitor 33 making a series inverse feedback circuit from thedouble-ended or balanced amplifier to a single-ended input circuit.

This connection is effected by the simple circuit arrangement whichincludes tapping the output winding of the amplier to include a portionof that winding in the cathode circuit of the driver stage. Obviously,the resistance of the kwinding included in the cathode circuit should berelatively low and may normally be provided by a relatively lowpotential tap on the winding. With this arrangement, a xed bias for theinverter stage and the driverstage is preferable, as indicated, in orderthat both cathodes may be grounded, the driver stagei operating aboveground potential by the amount of the feedback potential. 1

It will be Vnoted further that from the driver stage signals are applieddirectly tothe output amplifier tube 41 and indirectly to the amplifiertube 48 through the inverter stage 55. Thus, from the input circuit of`the amplifier, provided with inverse feedback, two stages ofamplification are included on one side between the input circuit and theoutput circuit from which inverse feedback potentials are derived, andthree stages are included on the opposite side, signals applied to theinput circuit 31-39 being conveyed through the tube 35, the tube 56 andthe tube 4S.

It is obvious that there is a greater possibility for phase shift in thethree-stage branch of the circuit than Ain the two-stage branch and,since both branches are coupled to the feedback winding 8e, it wouldseem that the resultant phase shift might seriously affect the operationof the feedback circuit. However, such is not the case, as has beenfound in actual operation, as the output voltage es is the resultant ofthe two voltages e1 and e2 amplified `through the output stage.

This may be represented in Fig. 2 in which the output load provided bythe loudspeaker or other device is reflected back through the outputtransformer 13 into the output circuit, as represented by the dottedrectangle 90, and the plate impedance of the tubes 41 and 48 arerepresented at 9| and S2 respectively. The input voltages e1 and e2multiplied by the amplification factor of the tubes 41-48 are indicatedin the figure and result in a generated output voltage es, of which thevoltage es appears across the output circuit, as shown. The voltage ezhas a negative sign for the reason that it is 180 degrees out of phasewith the other voltage e1.

'I'his isv shown in Fig. 3 by the vector diagram of the input and outputvoltages and in which it will be noted that, while the phase distortionindicated by the angle 9 is greater for the threestage branch of thecircuit than for the twostage branch, the resultant voltage ea has lessphase, distortion than the three-stage branch, clearly indicating thefeasibility of operation of an inverse feedback amplifier having adiffering number of stages in one branch of a divided feedback loop thanin another branch.

Conditions are consequently better than for a normal three-stageamplifier, although heretofore it has been considered that the use offeedback with phase inversion represented a special problem because thegain to the grid 4B is the gain of the driver stage 35, while the gainto the grid 65 is the gain of the driver stage 35 multiplied by the gainof the inverter stage 56, where the gain of the latter stage is madeequal to unity, as by the bleeder or potential divider connection shown.

The phase shift to the grid 46 is the phase shift in the driver stage,while the phase shift for the grid E is the phase shift in the driverstage plus the phase shift in the phase inverter. The vector diagramanalysis, however, as above referred to, shows that the overall phaseshift referred to is less than that for a three-stage amplifier,although somewhat greater than for a two-stage amplifier.

It has been found that the input capacity of the input grid of theinverter tube 55 may cause the high frequency response to be lowered anda phase shift which may be corrected by including a portion of thesignal voltage bleeder or potentiometer device in a phase-correcting'network which includes a capacity of the value of that of the inverterstage input grid.

The object in correcting for phase shift and frequency loss is tostabilize the inverse feedback circuit, the stability of the circuitdepending upon the amount lof attenuation which may be obtained prior toor in advance of the 180- degree phase shift in the inverter stage. Inorder' to reach a stable condition of operation and thereby to makepossible a greater amount of degeneration in an inverse feedbackamplifier of the type shown providing a phase inverter, the highfrequency cut-off has been separated as much as possible in the variousstages.

In the input circuit of the inverter, the phase shift is compensated orcontrolled by a cornpensating capacitor 94 connected in parallel withthe high potential section 5I of the inverter bleeder resistor andhaving Va predetermined relation to the input capacity of the invertertube 55.

Referring to Fig. 4 and Fig. l, with the capacitor at 94 in shunt withthe high potential section 5| of the bleeder and with the tube inputcapacity indicated at 95 across the second resistor section 52, the highfrequency response in the inverter stage may be extended and the phaseshift minimized by making the product R1C1=R2C2, where R1 and Rz equalthe resistances of the resistors 5| and 52, respectively, and C2 and C1equal the input capacitance 95 and the compensating capacitance 94,respectively.

If the capacitor S4 is omitted, the effective input -capacitance C2 ofthe inverter stage, together with the voltage regulation of resistances5I and 52, will cause a high frequency cut-olf together with a negativephase shift in the signal potential transmitted through the inverterstage. However, with the capacitor 94 alone in the circuit, neglectingthe effect of the input capacitance of the tube 55, a high frequencygain or increase in signal response will be obtained, together with apositive phase -shift in the signals transmitted through the inverterstage.

A combination of the two opposing effects may thus be obtained,involving values of resistance and capacitance in accordance with theformula above referred to, which may cause substantially zero signalattenuation and phase shift Within the high frequency end of the audiofrequency signal range of the amplifier. If the value of the capacitor94 is increased above an optimum value for substantially zero highfrequency signal attenuation and phase shift, an increase in the highfrequency response may be obtained Ywith a slight positive phase shift.

In the amplifier shown, for a coupling and phase inverter networkbetween two RCA 6J5 tubesA at 35 and 55 and two RCA GF6 pentode tubes at4l' and 48, the grid coupling resistors 62 and 53may each have a valueof substantially 270,000 ohms with a bypass capacitor B6 of .25 mfd. Theanode coupling resistors 60 and 61 may have a value of 100,000 ohms witha coupling capacitor 45 of .l mfd. and a coupling capacitor at 64 of.025 mfd. The input resistor has a resistance of 1 megohm, with a bypasscapacitor at 38 of .5 mfd.

In the inverter bleeder circuit, the anode potential blocking capacitor50 may have a value of .01 mfd., the compensating capacitor 94 a valueof 4.7 mmfd., and the resistor sections 5l-'52 may have values,respectively, of 560,000 ohms and 47,000 ohms.

In order to prevent undesired high and low frequency oscillations in theamplifier, resulting from a phase shift in signal voltages transmittedthrough the amplifier in a direction normally providing inversefeedback, for example, a phase shift of substantially degrees andregenerative feedback, the coupling network in the two branches of thefeedback loop is modified to effect a high frequency cut-olf in onebranch and a low frequency cut-off in the other branch. This isaccomplished in the circuit by connecting in shunt with the signalcircuit in the twostage branch a capacitor 96 from the output circuit 53to ground, the connection preferably being made to the anode circuit 53of the driver stage 35, as shown, in connection with the lead 49. In thethree-stage branch between the inverter stage 56 and the output tube 4B,the coupling capacitor 64 is reduced in value from .1 mfd., as in thetwo-stage branch at 45, to a low value sufficient to appreciablyattenuate low signal frequencies, particularly adjacent to and below theuseful audiofrequency range and may have a value of .025 mfd., ashereinbefore referred to, for this purpose. The capacitor 96 mtr have avalue in the present example of .005 m With this arrangement in thebranched inverse feedback circuit, continuous oscillation at relativelyhigh frequencies of the order of 20,000 cycles and above, and atrelatively low frequencies substantially below the useful range ofaudibility, resulting in excessive distortion and overload of theamplifier, is effectively prevented.

By providing a high frequency cut-off in the plate circuit of the driverstage at as low a fre- -the tendency of said amplier to'` oscillateabove quency as is consistent with a satisfactory overall frequencyresponse, and by extending the high frequency response range in all ofthe other signal conveying circuits of the amplifier, the oscillationsreferred to are prevented, while at the same time the inverse rfeedbackcharacteristic of the amplifier serves to correct for the high frequencysignal attenuation injected into the one branch of the feedback loop.Like-wise, this inverse feedback action serves to correct for the lowfrequency attenuation in the other three-stage branch of the inversefeedback loop.

It is this characteristics of inverse feedback amplifiers tending tomaintain an overall uniform frequency response which also operatesadversely with regard to anytone-control action which may be desired inthe circuit. Accordingly, it will be noted that 'tone-control, meansapplied to the volume-control circuit 2f225 is included outside of theinverse feedback loop of the amplifier, and. for manual adjustment maybe provided in the intermediate frequency amplifier, as hereinbeforereferred to, so that the signal fidelity prior to amplification in theinverse feedback amplifier may be controlled.

For example, by controlling the coupling between the intermediatefrequency amplifier tuned circuits and the input circuits therefor, byadjustment of the coupling windings I3 and i 1i shown in Fig. l, thesignal attenuation about the intermediate frequency of 455 kes. may becontrolled to provide variation in the selectivity responsecharacteristic, as indicated at 97 in Fig. 5. This may be made to resultin attenuation of the high frequency signals in the audio frequencyoutput of the iirst `detector and amplifier 8 in accordarice with theresponse curves indicated at 98 in Fig. 6, the lower audio frequencycut-off corresponding to the narrow frequency attenuation curves in Fig.5, as will be understood from a comparison of Figs. and 6. By thismeans, the signal potential e, applied to the input circuit of thedriver stage or iirst stage audio frequency ampliiier within thefeedback loop may be controlled independent of the inverse feedbackfrequency corrective action, thereby resulting inan effective tonecontrol for the receiving systei We claim as our invention:

1. An inverse feedback amplifier comprising, in combination, a driverstage, a phase inverter stage, a balanced output amplifier stageimpedance coupled to said first and second-named stages and having aninverse feedback connection with said driver stage, means including acoupling network between said driver stage and said inverter stage forapplying a predetermined value of signal voltage to said inverter stage,and means in saidlast-named network for compensating the inputcapacitance of said inverter stage thereby to prevent phase shift in thefeedback loop of the amplifier through said inverter stage.

2. In an inverse feedback amplifier having an input circuit and anoutput circuit, the combination of means' providing an inverse feedbackcircuit between said output circuit and said input circuit, a pluralityof amplier stages interposed between said input circuit and said outputcircuit including coupling means for transmitting signals through saidamplifier, said signal transmitting means including an interstagecoupling network for reducing the overall phase shift of signalstransmitted through said amplifier, and said amplifier further includingan interstage coupling network having coupling elements for reducing andbelow a predetermined frequency range.

3. An inversev feedback amplifier comprising, in combination, a driverstage, a phase inverter stage, a balanced output amplifier stageimpedance coupled to said first and second-named stages and having aninverse feedback connection with said driver stage, means including acoupling network between said driver stage and'said inverter stage forapplying a predetermined value f of signal voltage to said inverterstage, means in said last-named network for compensatingy the inputcapacitance of said inverter stage thereby to prevent phase shift in thefeedbackl loop of the ampliiier through said inverter stage, meansincluded in the coupling between said balanced output amplifier stageand said phase inverter stage for preventing oscillations in theamplifier below a predetermined low frequency, and means in the couplingbetween said balanced output amplifier stage and the driver stage forpreventing oscillations in the amplifier above a predetermined highfrequency.

4. An inverse feedback amplier comprising, in combination, a balancedampliiier stage having an output circuit, a driver stage impedancecoupled to one half of said output stage, an inverter stage impedancecoupled to the other half of said output stage, said impedance couplingincluding means for preventing oscillations in said vampliiier above andbelow the operating frequency range of the amplifier, means providing aninverse feedback circuit between said output circuit and the driverstage, means providing a coupling network between said driver stage andsaid inverter stage including a pair of series-connected voltage bleederresistors to the junction of which said inverter stage is connected forapplying thereto a predetermined signal potential, and means in saidnetwork for compensating the input capacitance of said inverter stage.

5. In an audio frequency amplifier, the combination of a driver stage, aphase inverter stage including an amplifier tube having a signal inputgrid and a predetermined input grid capacitance, means providing acoupling network between said driver stage and inverter stage includinga pair -of series-connected voltage' divider resistors for said inverterstage, means providing a connection for said signal input grid with thejunction of said resistors, a compensating capacitor connected inparallel with the initial one of said series resistors and having apredetermined capacity relation to the input capacitance of saidinverter stage, whereby distortion in said network at high frequenciesis minimized, and means providing an inverse feedback connection to saiddriver stage. Y

6. In an audio frequency amplifier having a signal input circuit, asignal output circuit and a loudspeaker device in said output circuit,the combination of means providing a balanced output stage including apair of pentode amplifier tubes coupled to said output circuit, a driverstage including an amplifier tube coupled to said input circuit and toone of the tubes in the output stage, a phase inverter stage includingan amplifier tube providing coupling means between said driver stage andthe other of said output stage amplifier tubes, means providing a seriesinverse feedback circuit between said output circuit and said driverstage providing correction for motional impedance reflected back fromsaid loudspeaker device to said output circuit, and means providing asignal bleeder coupling network between said driver stage and theinverter stage including a resistance and capacitance element forpreventing phase shift and high frequency signal attenuation in saidbleeder network.

7. In an inverse feedback amplifier having an input circuit and anoutput circuit, the combination of a feedback circuit between said inputand output circuits, a driver stage, a phase inverter stage, and abalanced output stage and interstage coupling means therefor providing adegenerative loop over two stages on one side of said amplifier and adegenerative loop over three stages on the other side of said amplifier,the overall phase shift through which is less than for a three-stageamplifier, means providing a signal bleeder circuit for said phaseinverter stage including circuit elements for preventing phase shift inthe inverter stage by reason of the input capacitance thereof, and meansin the coupling network for the first-named degenerative loop forpreventing v oscillations above a predetermined high frequency, andmeans in the other degenerative loop of the amplier for preventingoscillations below a predetermined low frequency.

8. In an audio frequency signal amplifier, the Y combination of a driverstage, a phase inver-ter stage, a balanced pentode amplifier outputstage resistance coupled to said first and secondnamed stages, a signaloutput circuit providing a frequency variable load on said output stage,means providing inverse feedback between said output circuit and saiddriver stage to lower the effective output impedance of said pentodeamplier stage, and thereby to eliminate compensatory loading of saidoutput stage, and means for preventing phase distortion between thedriver stage and the inverter stage by reason of the input capacitanceof the inverter stage.

9. The combination with an amplifier comprising a driver stage, a phaseinverter stage and ,v

a balanced amplifier output stage resistance coupled to said rst andsecond-named stages, a

signal output circuit for said balanced output stage, said first-namedstage including a driver tube having a cathode connected to saidamplifier output circuit through an inverse feedback circuit, wherebythe inverse feedback loop through the amplifier comprises two branchcircuits, means for preventing phase distortion between the driver stageand the inver-ter stage by Vreason of the input capacitance of theinverter stage, means in one of said branch circuits for preventingoscillations in said amplifier above a predetermined high frequency, andmeans in the other branch circuit for preventing oscillations in a rangebelow a predetermined low frequency.

10. In an amplifier, the combination of a driver stage, a phase inverterstage, a balanced output amplifier stage impedance coupled to said rstand second-named stages and having an inverse feedback connection withsaid driver Stage, means including -a coupling network between saiddriver stage. and said inverter stage for applying a predetermined valueof signal voltage to said inverter stage, means in said lastnamednetwork for compensating the input capacitance of said inverter stagethereby to fix the frequency characteristic of the amplifier and toprevent phase shift in the feedback loop of the amplifier through saidinverter stage, and means outside of the feedback loop of said amplifierfor modifying the frequency characteristic of signals transmittedthrough said amplifier to further enhance the tone quality of saidsignals.

l1. In an amplifier, the combination of a driver stage, -a phaseinverter stage, a balanced output amplifier stage impedance coupled tosaid vfirst and second-named stages and having an inverse feed-backconnection with said driver stage, means including acoupling networkbetween said driver stage and said inverter stage for applying apredetermined value of signal voltage to said inverter stage, means insaid lastnamed network Y for compensating the input capacitance of saidinverter stage thereby to prevent phase shift in the feedback loop ofthe amplifier through said inverter stage, means within the feedbackloop for attenuating signals in frequency ranges above and below theoperating range of the amplier, whereby the frequency characteristic ofthe amplifier is xed, and control means outside of the feedback loop ofv said amplifier for modifying the frequency characteristic of signalstransmitted through said amplifier to further enhance the tone qualityof said signals.

PAUL F; G. HOLST.

LOREN R. KIRKWOOD.

